(1) Filed of the Invention
The present invention relates to a preform for a composite material forming a composite material by compounding a light metal such as an aluminum alloy, an aluminum composite material formed by impregnating a hot solution of an aluminum alloy to the preform for the composite material, and a manufacturing the same.
(2) Description of the Background Art
A part made of a light metal such as aluminum which is excellent to lower its weight and to provide high durability and high thermal expansion coefficient property, for example, in an automobile to increase such fuel efficiency and stable running is increasing. Some part such as a piston part of an engine composing the automobile is being used under severe condition including such as pressure and temperature to improve fuel efficiency and lower hazardous substances in the exhaust. For example, a piston thread portion formed in the piston of the engine which is under severe use condition requires high abrasion resistant property and strength. Accordingly, application of an aluminum composite material made of an aluminum alloy reinforced by a reinforcement material such as ceramics is increasing in such part.
Such aluminum composite material formed by impregnating the hot solution of the aluminum alloy to the preform for the composite material pre-composed of a reinforcement material of such as particle, short fiber and whisker is well known. For example, when only a certain portion of the part is reinforced, the structural part is integrally formed by cast forming in which the hot solution of the aluminum alloy is pored into the metal mold after the preform for the composite material is installed at the potion where the certain portion is formed. For example, when the piston in which the above piston thread portion is reinforced is formed, the piston in which the thread portion is reinforced is integrally formed by casing the hot solution of the aluminum alloy with high pressure after the preform for the composite material using aluminum borate whisker as the reinforcement material is installed at the piston ring thread portion forming position of the casting metal mold for the piston. Aluminum borate whisker is being optimally used because it has excellent abrasion resistant and strong property and is relatively inexpensive.
For example, the preform for the composite material composed of the aluminum borate whisker can be formed by sintering at approximately 1200° C. after stirring the aluminum borate whisker and an alumina sol as inorganic binder in water and suctioning water of the aqueous mixture using a certain filter, which is a general method to form a preform for a composite material. In the preform for the composite material, each adjacent aluminum borate whisker is bound respectively by gelation of the alumina sol and crystallization.
On the other hand, the preform for the composite material composed of the aluminum borate whisker is disclosed in Japanese Laid Open Patent Publication H9-316566, which is formed by adding an inorganic binder (alumina sol) to aqueous mixture of a ceramics particle (titanium oxide), an aluminum borate whisker, a potassium titanate whisker, a short fiber (alumina fiber), and powder with sintering property (such as graphite) followed by sintering at certain temperature after suctioning water. The preform for the composite material has a structure in which the ceramics particle adheres on the surface of the aluminum borate whisker. A potassium titanate whisker has a superior sintering property and eases directly or indirectly to bind the ceramic particle and the aluminum borate whisker by sintering with them. Further, the binding force which can hold such binding status is performed by action of the inorganic binder on sintering the ceramic particle and the aluminum borate whisker. Then, the aluminum composite material having an excellent abrasion resistant property is formed by impregnating a hot solution of an aluminum alloy to the preform for the composite material. Further, volumetric percentage of the potassium titanate whisker is adjusted less than 5% because the potassium titanate whisker softens the preform for the composite material and acts to lower its strength.
Meanwhile even though the inorganic binder is added and the sintering temperature is set to relatively high approximately 1200° C., the preform for the composite material formed from the aluminum borate whisker and the inorganic binder cannot be strong sufficiently because the binding force of each whisker is weak. Accordingly, when cast forming in which the hot solution of the aluminum alloy is impregnated is carried out at relatively high pressure to increase impregnating property of the hot solution, problematic deformation and break take place easily. The aluminum composite material formed from such preform for the composite material having insufficient strength could not satisfy required property for an engine piston.
On the other hand the preform for the composite material comprising the aluminum borate whisker formed based on Japanese Laid Open Patent Publication H9-316566 had limited strength because the binding force sintering the aluminum borate whisker, the ceramics particle and the potassium titanate whisker depended mainly on crystallization of the inorganic binder. Accordingly there was limitation to increase productivity because it was difficult to use high impregnation pressure and increase impregnation rate on cast forming. Further, the abrasion resistant property increased by addition of the aluminum borate whisker was possibly lowered because the potassium titanate whisker was relatively soft. Further, in such preform for the composite material, a powder with burnout property was added to increase impregnating property of the hot solution and during sintering the powder with burnout property was burnt out to increase breathability and generally graphite or activated charcoal was used as the powder with burnout property. Nevertheless the aluminum composite material in which graphite or activated charcoal was burnt out had limitation in performing sliding property and vibration damping property because graphite and activated charcoal was active to increase sliding property and vibration damping property of the aluminum composite material.